1. Field of the Invention
The present invention relates to a hydraulic control apparatus for an automatic transmission that is mounted, for example, in a vehicle, and in particular, relates to a hydraulic control apparatus for an automatic transmission that establishes a reverse speed by outputting an engagement pressure to the hydraulic servo of a friction engagement element by a solenoid valve that is energized when traveling in reverse during normal operation.
2. Description of the Related Art
Conventionally, a staged automatic transmission that is mounted, for example, in a vehicle, enables establishing each shift speed during forward travel and a reverse speed by controlling the engagement state of a plurality of friction engagement elements (clutches and brakes) by using a hydraulic control apparatus and establishing a transmission path in the speed change mechanism for each shift speed. In such a hydraulic control apparatus, a plurality of solenoid valves are provided that regulate and output engagement pressures to respective hydraulic servos that releasably engage the plurality of friction engagement elements, and the control of the multi-stage shifting described above is carried out by engaging the friction engagement elements necessary to establish the shift speeds by using the electronic control of these solenoid valves (refer, for example, to Japanese Patent Application Publication No. JP-A-8-42681).
However, in the hydraulic control apparatus described above, in order to prevent the friction engagement elements from unintentionally engaging when, for example, a failure (damage) occurs, or in order to suppress electrical consumption during travel, preferably a normally closed type valve, which does not output oil pressure when not energized, is used for the solenoid valves described above. Thus, during normal operation in such a hydraulic control apparatus, when the shift range is changed to the forward range or the reverse range based on the operation of the shift lever, the necessary solenoid valves are energized and engagement pressures are supplied to the hydraulic servos of the friction engagement elements that engage in the forward shift speeds or the reverse shift speed, respectively.
However, in the hydraulic control apparatus described above, when the solenoid valves that are necessary for establishing the reverse speed remain de-energized due to, for example, some sort of failure, there is a problem in that the reverse speed is not established, that is, the reverse travel of the vehicle becomes impossible.
In such fail states in which the solenoid valves that are necessary for reverse speed are not energized, the following cases can be considered: the case in which a sensor that detects, for example, the shift range, fails, the shift range cannot be detected, and whether any of the solenoid valves should be energized cannot be determined, and the case in which, for example, a severed wire or short occurs and an all-solenoids-off failure mode is set. In this all-solenoids-off failure mode, all of the solenoid valves are de-energized in order to prevent the friction engagement elements from engaging unintentionally when any sort of failure has been detected.